The goals of this project are twofold: 1) the characterization of C1qR expression on human polymorphonuclear phagocytes (PMN) and monocytes, and 2) investigation into the modulation of IgG- and complement-mediated phagocytosis by C1q. C1q is a ligand for cell surface receptors on a variety of cell types. In the last year we have further characterized the C1q ligand-receptor interaction on human PMN and monocytes. We have shown that the chemoattractant n-formylleucylphenylalanine (fMLP), and the phorbol ester phorbol dibutyrate (PDBu) do not upregulate C1qR number on PMN and monocytes. This is in contrast to the behavior of other complement receptors (CR1, the C3b receptor; CR3, the iC3b receptor) presented with the same stimuli. We are presently investigating the possibility that the C1qR is internalized during response to fMLP and PDBu. In the last year, we have also found that solid phase C1q can greatly enhance Fc receptor mediated phagocytosis in human monocytes and culture-derived macrophages. This effect is dose responsive, can be blocked by anti-C1q, and seems to be mediated via the collagen-like tail region of the C1q molecule; the same region of the molecule known to interact with the C1qR. Present studies involve the further characterization of this response and its possible relation to that effect seen with the extracellular matrix proteins laminin and fibronectin.